The optical properties of electrochromic materials change in response to electrically driven changes in oxidation state. Thus, when an applied voltage from an external power supply causes electrons to flow to (reduction) or from (oxidation) an electrochromic material, its transmittance properties change. In order to maintain charge neutrality, a charge balancing flow of ions in the electrochromic device is needed. By enabling the required electron and ion flows to occur, an electrochromic device utilizes reversible oxidation and reduction reactions to achieve optical switching.
Conventional electrochromic devices comprise at least one thin film of a persistent electrochromic material, i.e., a material which, in response to application of an electric field of given polarity, changes from a high-transmittance, non-absorbing state to a low-transmittance, absorbing or reflecting state. Since the degree of optical modulation is directly proportional to the current flow induced by the applied voltage, electrochromic devices demonstrate light transmission tunability between high-transmittance and low-transmittance states. In addition, these devices exhibit long-term retention of a chosen optical state, requiring no power consumption to maintain that optical state. Optical switching occurs when an electric field of reversed polarity is applied.
To facilitate the aforementioned ion and electron flows, an electrochromic film which is both an ionic and electronic conductor is in ion-conductive contact, preferably direct physical contact, with an ion-conducting material layer. The ion-conducting material may be inorganic or organic, solid, liquid or gel, and is preferably an organic polymer. The electrochromic film(s) and ion-conductive material are disposed between two electrodes, forming a laminated cell.
When the electrode adjacent to the electrochromic film is the cathode, application of an electric field causes darkening of the film. Reversing the polarity causes electrochromic switching, and the film reverts to its high-transmittance state. Typically, an electrochromic film such as tungsten oxide is deposited on a substrate coated with an electroconductive film such as tin oxide or indium tin oxide to form one electrode. The counter electrode is typically a similar tin oxide or indium tin oxide coated substrate.
Since an electrochromic device may be modeled as a non-linear passive device having an impedance dominated by a capacitive component, the amount of charge transferred to an electrochromic device is typically controlled by utilizing current sources and current sinks.
In a known arrangement for controlling an EC device, an up/down counter is responsive to an up/down signal and a clock signal to produce a digital word representative of a desired EC charge level. Control logic is used to convert the digital word to a current source/sink programming signal suitable for causing a current source (or sink) to impart the desired charge level to the EC device.
Unfortunately, the above arrangement utilizes various components (e.g., current source and current sink transistors) having characteristics that tend to drift over time and temperature, thereby imparting more or less charge to the EC device than is otherwise indicated by the digital word produced by the up/down counter. In addition, EC devices themselves are subject to operational degradation over time and temperature. Moreover, the amount of energy required to charge an EC device is typically greater than the amount of energy required to discharge such a device. Thus, over a given period of time or temperature, an EC charge error may be accumulated such that the EC device may be significantly lighter or darker than desired.
Therefore it is seen to be desirable to provide a method and apparatus that corrects for system and device errors within an electrochromic control system such that an indicated desired EC charge level more closely comports with an actual EC charge level.